Originally Posted by Some guy at another forum
The answer to your question hinges on what you consider "good sound," or in other words, how much audio quality degradation you're willing to accept in exchange for file size economy. For some people, the whole idea of compressing a song in MP3 ( or any other "lossy" compression format) is barbaric and destructive. Other people aren't too particular and are unperturbed by the quite obvious audio artifacts in an MP3 file encoded at 128 kbps.
Short answer: if you're very particular about audio quality, then you will encode at the highest bit-rate possible, which in iTunes is 320 kbps, which will also yield the largest file sizes. If you're not too picky, the type of person who can't really tell or notice the difference between the original audio CD and a 128 kbps MP3, then you're free to experiment with lower bit rates (128, 160, 192, 256, etc). Personally, I encode at 160 or 192 kbps.
There's another factor to consider here: whether or not to turn on the option for Variable Bit Rate (VBR) encoding. The original MP3 spec employed Constant Bit Rate encoding, or CBR, and most people still use this today. CBR works by adjusting the audio quality to ensure a consistent and predictable output file size. Easy passages in songs (those with narrow stereo separation and less audio data, for example) get assigned more bits in CBR, while complex passages may get less bits than optimal. For example, a 128k (16k/sec) bit rate audio file that is 30 seconds in duration will consistently produce a 480k MP3 file. In brief, CBR sacrifices audio quality to ensure consistent output file size.
VBR is a newer development in the world of MP3. With VBR, the number of bits allotted to song passages varies with its complexity, ensuring the optimal bit rate for highest audio quality. In VBR, therefore, output file size cannot be predicted, because the bit rate varies. In general, VBR sacrifices file size to preserve audio quality. The downside to this is that you must check first if your MP3 player supports VBR-encoded files.
And what are the other compression formats in itunes?
iTunes can encode to MP3, AAC (Advanced Audio Coding) which is the format of the iTunes Music Store, AIFF, Apple Lossless format, and WAV.
AAC - a newer and supposedly more efficient compression format than MP3, capable of producing equivalent audio quality at smaller file sizes. Downside: relatively few players support AAC, and only one player (iPod) supports protected AAC, which is the format of songs bought from the iTunes Music Store. If you have an iPod and don't foresee having to play your songs on a different device, then this shouldn't be a problem for you. AAC is probably the best way to go for most iPod owners.
AIFF and WAV - very large file sizes, roughly one minute = 10 mb. But much better audio quality than any highly compressed format such as MP3 or AAC.
Apple Lossless format - new in iTunes 4.5. Supposedly can bring a song to half of its uncompressed file size with no audio degradation, allowing you to double the number of songs on an audio CD, say. I haven't used this yet, so I can't really comment.
There are other compression formats not supported in iTunes, such as Ogg Vorbis (unlike all the other formats mentioned here, this one is free and open source, and supposedly at par with AAC/MPEG-4) and Windows Media Audio (WMA) (I will spare you the agony of linking to Microsoft's labyrinthine and hard-to-navigate web site). If you're going to stick with iTunes as your encoder, then there's no need to concern yourself with these other formats.
The question isn't really "which is best" but rather "which is best for you?"